silver-based nanoparticles for surface …phome.postech.ac.kr/user/pnel/publication/124....

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Organic optoelectronic devices including organic light-emitting diodes (OLEDs) and polymer solar cells (PSCs) have many advantages, including low-cost, mechanical fl exibility, and amenability to large-area fabrication based on printing techniques, and have therefore attracted attention as next-generation fl exible optoelectronic devices. Although almost 100% internal quantum effi ciency of OLEDs has been achieved by using phosphorescent emitters and optimizing device structures, the external quantum effi ciency (EQE) of OLEDs is still limited due to poor light extraction. Also, although intensive efforts to develop new conjugated polymers and device architec-tures have improved power conversion effi ciency (PCE) up to 8%–9%, device effi ciency must be improved to >10% for commercialization of PSCs. The surface plasmon resonance (SPR) effect of metal nanoparticles (NPs) can be an effective way to improve the extraction of light produced by decay of exci-tons in the emission layer and by absorption of incident light energy within the active layer. Silver (Ag) NPs are promising plasmonic materials due to a strong SPR peak and light-scattering effect. In this review, different SPR prop-erties of Ag NPs are introduced as a function of size, shape, and surrounding matrix, and review recent progress on application of the SPR effect of AgNPs to OLEDs and PSCs.

to 110 lm W −1 . [ 16,17 ] This rapid progress offers the possibility for OLEDs and PSCs to be next-generation fl exible organic opto-electronic devices with low cost and high effi ciency.

OLEDs have received great attention as potential next-generation displays due to their low power consumption, excel-lent color gamut, fast response time, and especially their fl exibility. [ 18 ] Although almost 100 % internal quantum effi ciency of OLEDs has been achieved by using phosphorescent emitters and optimized device structures, [ 19 ] the external quantum effi ciency (EQE) is still limited due to poor light extraction. [ 16,18,20 ] The key chal-lenge is to improve the extraction of light trapped between layers and the energy transfer between donor and acceptor mole-cules in emissive layers.

The power conversion effi ciency (PCE) improvement of PSCs is mostly dependent on the development of new conjugated polymers and device structures. At pre-sent, an important challenge is to fi nd effective ways to further improve the per-

formance of PSCs in conjunction with novel materials and device architectures. One simple strategy to achieve high device effi ciency is to increase the light absorption within the active layer by producing a thick active layer, thereby improving short-circuit current density. However, exciton diffusion length and charge-carrier mobility of organic materials are in the range of 1–10 nm and 10 −5 −10 cm 2 V −1 s −1 , respectively. [ 21–26 ] As a result, although a thick active layer can absorb many photons, consid-erable recombination loss can occur during exciton diffusion, dissociation, and charge transport. [ 27,28 ] Specifi cally, regulating the thickness of active layer entails a trade off between photo-generated charge-carrier effi ciency and collection effi ciency. [ 29 ] The key goal is to reduce the physical thickness of the active layer while maintaining its optical thickness. [ 30 ]

One useful strategy for light trapping and emitting in PSCs and OLEDs is to exploit the surface plasmon resonance (SPR) effect of metal nanostructures. Surface plasmons are collective oscillations of free electrons at the interface between the metal and dielectric. [ 31–33 ] The SPR effect, which is induced by an elec-tric fi eld at specifi c incident wavelength, leads to intense sur-face plasmon absorption bands and strong light scattering with concurrent strengthening of local electromagnetic fi elds. [ 30,34 ] By exploiting either localized surface plasmons (LSPs) excited in metal nanoparticles (NPs) or surface plasmon polaritons (SPPs) DOI: 10.1002/ppsc.201400117

Silver-Based Nanoparticles for Surface Plasmon Resonance in Organic Optoelectronics

Su-Hun Jeong , Hyosung Choi , Jin Young Kim ,* and Tae-Woo Lee*

1. Introduction

Organic optoelectronic devices including organic light-emitting diodes (OLEDs) and polymer solar cells (PSCs) are promising candidates for next-generation fl exible and stretchable opto-electronic devices. [ 1,2 ] The devices have attracted great attention due to advantages such as low cost, solution processability, and feasibility of large-area fabrication on fl exible and stretchable substrates. [ 3–6 ] Signifi cant efforts to synthesize new organic materials, to develop device architectures, and to control the morphology of organic layers, have improved the effi ciencies of PSCs to 10% [ 7–15 ] and the panel effi ciency of white OLEDs

S.-H. Jeong, Prof. T.-W. Lee Department of Materials Science and Engineering Pohang University of Science and Technology (POSTECH) Pohang, Gyungbuk 790–784 , South Korea E-mail: [email protected] H. Choi, Prof. J. Y. Kim Department of Energy Ulsan National Institute of Science and Technology (UNIST) Ulsan 689–798 , South Korea E-mail: [email protected]

Part. Part. Syst. Charact. 2014, DOI: 10.1002/ppsc.201400117

http://doi.wiley.com/10.1002/ppsc.201400117

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the light can be effi ciently concentrated into a thin active layer, thereby maximizing the light absorption. In particular, metal NPs function as effective subwavelength antennas and scat-tering elements to store the incident energy in LSP modes and to increase the optical path length of the incident light within the active layer, leading to photogeneration of charge-carriers in PSCs ( Figure 1 ). [ 29,30 ] In addition, metal NPs are easily appli-cable to OLEDs and PSCs by means of solution processing. [ 35–37 ]

In this review, we focus on an effi cient light-trapping and -emitting approach using the SPR effect of silver NPs (Ag NPs) to fabricate high-performance plasmonic OLEDs and PSCs because of many advantages of Ag NPs compared to other metal NPs.

2. Unique Properties of Nanoparticles

Ag NPs constitute a promising plasmonic nanomaterial because of their many advantages. We will discuss the merits of Ag NPs and their unique SPR effects depending on size, shape, elemental composition, and surround matrix in the next subsections.

2.1. Advantages of Silver Nanoparticles

Among various metal NPs, Ag NPs have exhibited the most effective light trapping potential due to their strong light scat-tering and surface plasmon (SP) strength. [ 38–40 ] In addition, SPR absorption spectra of silver nanoparticles (Ag NPs) can be controlled from 300 (ultraviolet (UV)) to 1200 nm (near-infrared (NIR)). The ability of metal NPs for SPR effect depends on its dielectric function ε including a real part ε r and an imaginary part ε i , both of which vary with excitation wavelength λ. [ 40 ]

The SPR effect of metal NPs with spherical structure can be described using the extinction (absorption + scattering) cross-section based on Mie theory, [ 41 ]

CR24

2ext

2 3m3/2

i

r m2

i2

π ελ

εε ε ε( )

=+ +

⎡

⎣⎢

⎤

⎦⎥

(1)

where C ext is the extinction cross-section, R is the NP radius, and ε m is the relative dielectric constant of the matrix sur-rounding the metal NPs. This equation implies that dielectric properties have strong effect on the interaction between light and metal NPs. In addition, the SP strength (or damping) of metal NPs can be expressed using the quality factor (QF), [ 42 ]

w d dwQF

/

2

r

i2

εε

( )( )

=

(2)

SP strength is proportional to QF. Specifi cally, high QF indi-cates strong plasmons and low QF means weak SP with a small C ext . Ag has higher QF than do other metals over the spectrum from 300 to 1200 nm. Interband transitions (IBTs), which are excitations of electrons from the conduction band to higher energy levels, are key factor for the SP strength. [ 43 ] In Ag, these transitions occur at much higher energies than the SPR,

whereas these transitions limit their SPR excitation to wave-length >500 nm for Au and >600 nm for Cu. [ 44,45 ] Taking into account, these factors and others including plasmonic ability,

Jin Young Kim is an associate professor in the Department of Energy Engineering, Ulsan National University of Science and Technology, Ulsan, Korea (July 2008–present). He received his Ph.D. in physics from Pusan National University (September 2000–February 2005). He served as an assistant research pro-fessor, in the Heeger Center

for Advanced Materials, Gwangju Institute of Science and Technology (GIST), Gwangju, Korea (July 2007–2008) and he was a postdoctoral researcher, in the Center for Polymers and Organic Solids, UC Santa Barbara, Santa Barbara, USA (April 2005–July 2007). His research interests include organic and hybrid solar cells, and energy-related devices.

Su-Hun Jeong received his B.S. in the Department of Materials Science and Engineering in 2012 from Pohang University of Science and Technology (POSTECH). He is a graduate student in POSTECH since 2010. His current research work is focused on polymeric electrodes for fl exible organic optoelectronic devices.

Tae-Woo Lee is an associate professor in the Department of the Materials Science and Engineering at POSTECH, Korea. He received his Ph.D. in chemical engineering from KAIST, Korea in February 2002. Then, he joined Bell Laboratories, USA as a postdoctoral researcher in 2002. From September 2003 to August 2008, he worked in

Samsung Advanced Institute of Technology as a member of research staff. He received a prestigious Korea Young Scientist Award from the President of Korea in 2008 and The Scientist of the Month Award from the ministry of sci-ence, ICT and future planning in 2013. His research focuses on printed and organic electronics based on organic and carbon materials for fl exible electronics, displays, solid-state lightings, and solar energy conversion devices.




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material cost, and chemical stability, Ag is the most economical and effective candidate for plasmonic applications ( Table 1 ). [ 40 ]

2.2. Photoluminescence Enhancement Mediated by Surface Plasmon Resonance of Silver Nanoparticles

Photoluminescence (PL) is a process in which a photoexcited material releases its energy as light. [ 46 ] Ag NPs have been reported to cause radiative enhancement when they have a certain size and are kept at a certain distance from emissive molecules. [ 47–51 ] This enhancement can come from enhanced resonance-energy transfer or from near-fi eld enhancement by the interactions of the emissive molecules with the SPR effect of Ag NPs in a certain wavelength of incident light. LSPs of Ag nanoparticles and propagating surface plasmons of Ag nanofi lms increase PL from a conjugated polymer, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV); changing the Ag nanostructures from NPs to nanofi lms by increasing the thickness of deposited Ag yielded large PL enhancement of a MEH-PPV fi lm. [ 52 ] Both surface localized and propagating surface plasmons contributed to the enhanced PL.

SPR-enhanced radiative emission can be used to enhance Förster resonance energy transfer (FRET) [ 47–49 ] between donor and acceptor molecules; In FRET, the energy of excited donor molecules is transferred to neighboring acceptor molecules by nonradiative dipole–dipole interaction ( Figure 2 ). [ 53,54 ] This energy transfer decreases the donor emission intensity, thereby reducing the donor lifetime, and enhances the acceptor emis-sion intensity. When the SPR wavelength of Ag NPs overlaps the wavelengths of donor emission and acceptor absorption, FRET from donor to acceptor molecules can be increased by

improving donor emission and acceptor absorption. [ 49,50 ]

The SPR effect-mediated energy transfer enhancement is greatly infl uenced by the shape, size and SPR wavelength of noble metal nanostructures, and by the posi-tion of the donor–acceptor pair relative to nanostructures. [ 49 ] The SPR effect of Au–Ag core–shell NRs affects FRET. [ 47 ] The energy transfer effi ciency E can be calculated from the energy transfer rate k ET and the sum k D of the donor’s radiative and non-radiative decay rates:

E

k

k kET

D ET

=+

(3)

When the SPR peak overlaps the donor emission peak, k D increases but k ET does not, so E decreases. When the SPR peak is between the donor emission peak and the acceptor absorp-tion peak, k ET increases and consequently E increases.

2.3. Surface Plasmon Resonance Effect of Silver Nanoparticles Depending on Size, Shape, Elemental Composition, and Surrounding Matrix

The SPR effect is collective oscillation of valance electrons that is induced by resonant photons when their frequency matches the natural frequency of surface electrons. Different noble metals have different resonant photon frequencies. Ag NPs have resonant wavelengths in UV–vis (vis) region. The reso-nant wavelength and SPR intensity of Ag NPs are dependent on their, [ 55,56 ] shape, [ 57–59 ] elemental composition (alloy, core–shell), [ 60–62 ] and surrounding matrix. [ 63–65 ]

The size of Ag NPs affects the SPR absorption peak and intensity as well as light scattering. For large Ag NPs, the scat-tering effect is more dominant than light absorption; further-more, increasing the size resulted in red-shift of the SPR peak and broadening of the absorption band ( Figure 3 a). [ 55,56 ] Ag nanocubes have the same tendencies as Ag NPs. Ag nanocubes exhibited continuous red-shift and broadening of SPR peaks as their edge length was increased from 36 to 172 nm. [ 66 ]

The shapes of Ag NPs strongly affect their plasmonic prop-erties. Changing the sharpness of the edges of Ag NPs affects their UV–vis extinction spectra [ 58 ] ( Figure 3 b). The SPR absorp-tion bands of Ag NPs steadily blue-shifts the Ag NP shape


Figure 1. Plasmonic light-trapping mechanism of metal NPs: a) light trapping by scattering from metal NPs at the surface of the device. b) Light trapping by the excitation of SPs in metal NPs embedded in semiconductor. Reproduced with permission. [ 29 ] Copyright 2010, NPG.

Table 1. Comparison of characteristics of various metals for plasmonic applications. Table reproduced with permission. [ 40 ] Copyright 2011, ACS.

Metal Cost [$/ounce] Plasmonic ability Nanostructure formations Chemical stability

Aluminum 0.049 Good in UV region Few Stable after surface passivation

Silver 13.4 Highest in QF Many Oxidation

Copper 14.8 IBT below 600 nm Few Easy oxidation

Palladium 265 Low QF Many Stable

Gold 950 IBT below 600 nm, high QF Many Very stable

Platinum 1207 Low QF Many Stable



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changes from triangular nanoplates to circular disks. Spe-cifi cally, Ag NPs with sharp edges have red-shifted extinction spectra compared to those of Ag NPs with rounded structures because the sharp nature of the Ag NPs tends to increase charge separation and to reduce the restoring force for the dipole oscillation.

The shapes (e.g., sphere, cube, octahedron, and right bipy-ramid) of Ag NPs also can affect their extinction spectra. [ 59 ] Ag spheres only have one SPR peak at 410 nm, whereas Ag cubes have a strong SPR peak at 450 nm and a few shoulders. Compared to Ag spheres, Ag cubes, octahedra, and right bipy-ramids have lower symmetry, leading to polarization of elec-trons in multiple directions. In short, the number of SPR peaks increases with the number of ways in which it can be polarized and the SPR peaks red-shift with increasing corner sharpness and anisotropy.

The geometry of Ag NPs can change the position of the SPR peak. The Ag NPs with sphere and cube structure have sharp dipole resonances that dominate the spectra. Compared to the SPR peak of Ag spheres, SPR peaks of Ag NPs with cube, octa-hedron, and right bipyramid structures are red-shifted about 40, 50, and 100 nm, respectively. Greater charge separation occurred by sharp corners of the cubic structure during dipole oscillation, leading to reduced restoring force and consequently longer resonance wavelength.

Alloying and developing core–shell structure are effec-tive strategies for tuning SPR properties of Ag NPs. Various Ag-based alloys can be synthesized by using different metal types and compositions such as Ag–Au, [ 67,68 ] Ag–Pt, [ 69,70 ] and

Ag–Pd. [ 69 ] One representative alloy structure is an Ag–Au alloy nanocage synthesized by galvanic reaction. [ 71 ] The SPR peak of these alloy nanocages can be tuned from visible to NIR by varying the Ag/Au ratio. Various types of core–shell metal NPs have been reported including Ag–Au, [ 72,73 ] Au–Ag, [ 74,75 ] and Ag-Cu. [ 76,77 ]

Core–shell metal NPs have unique SPR properties com-pared to mixtures of pure metal NPs. The mixture of Ag and Au NPs has SPR peaks at 400 and 520 nm, which correspond to the plasmonic peak of pure Ag NPs and pure Au NPs, respec-tively. [ 78 ] In contrast, Ag–Au core–shell metal NPs have only one SPR peak at 500 nm with a broad shoulder at 400 nm. [ 79 ] This core–shell structure can be used together with the strategy of controlling the shape of metal NCs. One example is use of Au–Ag core–shell nanorods (NRs). [ 62 ] The extinction spectra of Au–Ag core–shell NRs can be tailored from visible to NIR by varying the Au core size and the Ag shell thickness. Pure Au NRs have transverse and longitudinal plasmon resonance wave-lengths at 512 and 868 nm, respectively; increasing the thick-ness of Ag shell resulted in blue-shift of SPR peaks from Au NRs, and new resonance peaks from Ag shell appeared after the Ag shell reached a certain thickness ( Figure 3 c). These mul-tiple plasmonic peaks may be attributed to synergic effects of bimetallic and asymmetric properties of Au–Ag core–shell NRs. This strategy of coating Ag NPs with different materials can eliminate the reactivity and toxicity of Ag. [ 80,81 ]

The SPR properties of Ag NPs can be tuned by engineering the dielectric constant of the surrounding matrix. In general, small Ag NPs in air have an SPR peak at 400 nm. [ 30 ] Different surrounding matrixes (including SiO 2 , Si 3 N 4 , Si, TiO 2 ) cause the peak to red-shift over the entire 500–1000 nm due to the change of dielectric constant ( Figure 2 d). [ 65 ] Surrounding metal NPs with a dielectric layer hinders direct quenching between metal NPs and semiconductors, and is therefore a useful way to prevent exciton quenching. [ 61 ]

3. Recent Progress in Plasmonic Organic Light-Emitting Diodes using Silver Nanoparticles

OLEDs have received great attention as a next-generation display due to their notable properties, especially fl exibility. [ 82 ] Although almost 100% internal quantum effi ciency of OLEDs has been achieved by using phosphorescent emitters and optimizing device structures, [ 19 ] EQE is still limited due to the poor light extraction. [ 20 ] The large difference between the refractive indices n of the air ( n = 1), glass ( n ≈ 1.5), indium tin oxide (ITO) ( n ≈ 2.0), and organic layer ( n ≈ 1.7–2) trap the emitted light in the ITO/organic layer and ITO/glass as wave guide modes. The large refractive index differences of layers trap the emitted light inside OLEDs. [ 83 ] Coupled with excitons, surface plasmons can cause increases in both radiation and non-radiative loss, depending on the size, interparticle distance of noble metal nanostructures, and spacing between noble metal nanostructure and emissive molecules. [ 84 ] When the distance between emissive molecules and the noble metal nanostructure is too small, non-radiative quenching of excitons occurs at the metal surface. Therefore, to maximize radiative enhancement by coupling between exci-tons and surface plasmons in OLEDs, the environment in which


Figure 2. Schematic of Förster resonance energy transfer (FRET). Repro-duced with permission. [ 54 ] Copyright 2012, The Royal Society of Chemistry.



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noble metal nanostructures incorporate with emissive molecules in OLEDs should be optimized. Recently, the SPR effect of Ag NPs has been reported as a tool for improving the light emission and energy transfer in OLEDs. [ 49,50,85,86 ]

3.1. The Surface Plasmon Resonance Effect of Silver Nanoparticles-meditated Energy Transfer in OLEDs

The near-fi eld enhancement by the SPR effect of Ag nanostruc-tures can improve the energy transfer in OLEDs by enhancing the absorbance of donor molecules and emis-sion of acceptor molecules. Electrolumi-nescence in a top-emitting OLED has been increased by a factor of 10 by exploiting SPR effect-mediated radiative energy transfer through a 1D periodically corrugated Ag cathode SPR effect ( Figure 4 a). [ 85 ] The peri-odically corrugated Ag cathode was intro-duced by using holography lithography to create corrugations on a silica substrate, then depositing the anode, organic layers, and the Ag cathode. Then a dye-doped dielectric acceptor layer was deposited on top of the one-dimensionally corrugated Ag cathode. The acceptor layer was excited by the electro-luminescence of a donor layer on the other

side of the cathode. The energy transfer from the donor to acceptor was enhanced due to the coupled surface plasmons from the periodic corrugated Ag surface. Excitons generated in the emitting layer emit photons into surface plasmons at the interface between the corrugated Ag cathode and electron-transporting layer. These surface plasmons couple with the sur-face plasmons at the other interface between the Ag cathode and the acceptor layer. The acceptor is excited by the surface plasmons and releases its energy.

The SPR effect of Ag NPs can also be used to enhance energy transfer between donor and acceptor molecules in


Decreasing edge sharpness

Increasing the thickness of Ag shell

Ext

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400 600 800 1000Wavelength (nm)

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ext

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on

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a

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ion

400 600 800 1000Wavelength (nm)

0.0

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1.0b

c d

Figure 3. The change in plasmonic absorption properties of Ag NPs as a function of conditions: a–c) Extinction spectra of a) size-controlled Ag NPs (13–94 nm in diameter), b) Ag NPs with different edge sharpness (from Ag triangular nanoplates to circular disk), and c) Au–Ag core–shell nanorods with different thickness of Ag shell. d) Scattering cross-section spectra of Ag NPs (100 nm in diameter) embedded in different dielectric layers. Repro-duced with permission: panel (a), [ 55 ] Copyright 2013, NPG; panel (b) [ 58 ] Copyright 2010, Wiley and (c), [ 62 ] Copyright 2012, Wiley; panel (d), [ 30 ] Copyright 2009, AIP.

Figure 4. a) Schematic structure of the corrugated OLED with a one dimensionally periodic silver cathode, and b) transmission electron image (TEM) images of Ag nanoclusters by thermal evaporation. Reproduced with permission: panel (a), [ 85 ] Copyright 2008, American Institute of Physics; panel (b), [ 50 ] Copyright 2009, OSA.



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OLEDs by exploiting the SPR effect of Ag NP-mediated FRET in fl uorescent OLEDs that use tris-(8-hydroxyquinolinato) alu-minum (Alq3) as a donor and 4-(dicyanomethylene)-2-methyl-6-(p-dimethyl-aminostyryl)-4H-pyran (DCM) as an acceptor. [ 50 ] To form a layer of Ag NPs, a very thin (<1 nm) Ag layer was deposited by thermal evaporation, and the size and spacing of Ag NPs was controlled by adjusting the mass thickness ( Figure 4 b). When the Ag NP with the SPR wavelength close to the wavelength of the donor emission were introduced, the donor decay rate increased. Additionally, when the SPR excita-tion energy approached the absorption range of the acceptor, the energy transfer between donor and acceptor was enhanced.

3.2. The Surface Plasmon Resonance Effect of Silver Nanoparticle-meditated Light Emission of OLEDs

Use of an LiF spacer can affect the interaction between the spon-taneous exciton emission rate of a donor and the SPR effect of Ag NPs ( Figure 5 a). [ 49 ] Ag NP fi lms were formed using thermal deposition, and Ag NPs of various size and density were depos-ited while controlling deposition rate and deposition thickness. The sample with the highest density had the highest PL emis-sion intensity because of the dependency of the SPR effect on Ag NP cluster size and density ( Figure 5 b). To confi rm the feasibility of the cathode structure, OLEDs with the following layer structure were fabricated using thermal vacuum evapo-ration: ITO/ N,N ′-di(naphtha-2-yl)- N,N ′-diphenyl-benzidine/Alq 3 /LiF/Ag cluster/LiF/Al. The OLED with LiF/Ag cluster/LiF-coated Al cathode showed slightly lower performance than

the conventional LiF/Al cathode device due to the poor carrier injection caused by the high work function of LiF/Ag contact. However, the result showed the feasibility of utilizing surface plasmons.

Recently, highly effi cient polymer light-emitting diodes (PLEDs) using Ag NPs have been reported. [ 87 ] Carbon dot-sup-ported Ag NPs (CD-Ag NPs) were prepared using CD as both a template and reducing agent for Ag NPs and then spin-cast underneath conducting polymer, poly(3,4-ethylenedioxythiophene):polystyrene sulfonic acid (PEDOT:PSS). The ensemble plasmon coupling effect from clustering Ag NPs in CD-Ag NPs contributed to improving the radiative emission. The maximum luminous effi ciencies of these PLED devices using poly(p-phe-nylene vinylene) copolymer, and Super Yellow (SY) for the emis-sion layer were increased 6.33 lm W −1 to 18.54 lm W −1 after Ag NPs were incorporated with a conducting PEDOT:PSS anode ( Figure 5 c). These results were confi rmed by the coincidence of the enhancements in the steady-state PL spectra and PL decay profi le. Compared to the PEDOT:PSS/SY fi lm without Ag NPs, that with Ag Ps had ≈30% higher PL intensity ( Figure 5 d), and longer average PL lifetime ( Figure 5 e).

4. Recent Progress in Plasmonic Polymer Solar Cells using Silver Nanoparticles

The performances of plasmonic PSCs have been gradually improved ( Table 2 ) by employing different Ag NPs including nanoplates, nanoprisms, alloy NPs, and core–shell NPs, and incorporating these NPs into the electrode, charge transport


Figure 5. Emission properties of plasmonic OLEDs with Ag NPs: a) Schematic structure of the LiF/Al nanocluster/LiF cathode (inset: TEM image of Ag clusters on the LiF fi lm). b) Photoluminescence spectra of samples 1–3 with the different Ag nanocluster densities. c) Luminous effi ciency of PLEDs with and without CD-Ag NPs. The inset of Figure 5 c is the device structure of plasmonic PLED with CD-Ag NPs. d) Steady-state photoluminescence spectra e) and photoluminescence decay profi le of the super yellow fi lm with and without CD-AgNPs. Reproduced with permission: panel (a) and (b), [ 49 ] Copyright 2009, American Institute of Physics; panel (c–e), [ 86 ] Copyright 2013, NPG.



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layer, active layer, and the interface between them. Ag NPs with different sizes, shapes, structures, and elemental compositions have been explored to develop high-performance plasmonic PSCs. Because of solution-processability of Ag NPs, they can be easily applied to diverse positions within the devices, such as inside various layers (electrode, charge transport layer, and active layer) or at the interfaces between them ( Figure 6 a).

4.1. Silver Nanoparticles Inside the Layers

One approach has been reported for developing an ITO-free electrode using Ag NPs. Ag NPs were incorporated into PEDOT:PSS fi lm to make a transparent conducting electrode instead of an ITO electrode and to use the SPR effect of Ag NPs at the same time. [ 6 ] The PCEs of these devices using a blend of poly(5,6-bis(octyloxy)-4-(thiophen-2-yl)benzo[c][1,2,5]thia-diazole) (PTBT) and [6,6]-phenyl-C 60 -butyric acid methyl ester (PC 60 BM) as the active layer were increased from 3.27% to 4.31% by introducing Ag NPs into the conducting PEDOT:PSS anode ( Figure 6 b). These Ag NPs contributed to the increase of electrical conductivity of PEDOT:PSS fi lm and to the increase in light absorption by the active layer; these processes were confi rmed by the coincidence of the enhancements in incident photon-to-current effi ciency (IPCE) and absorption ( Figure 6 c).

Size-controlled Ag NPs were incorporated into a PEDOT:PSS hole transport layer by mixing Ag NPs and PEDOT:PSS and spin-casting this solution on top of an ITO substrate ( Figure 6 d). [ 55 ] Large Ag NPs have a tendency to increase both the ratio of total scattering power to the total absorption power, and the ratio of the forward scattering to the total scattering ( Figure 6 e). Ag NPs with a diameter

of 67 nm in the PEDOT:PSS layer resulted in remarkable increases in PCEs from 6.4% to 7.6% for poly[N-9-hepta-decanyl-2,7-carbazolealt-5,5-(4,7-di-2-thienyl-2,1,3-benzothia-diazole)] (PCDTBT): [6,6]-phenyl-C 70 -butyric acid methyl ester (PC 70 BM) devices, and from 7.9% to 8.6% for polythieno[3,4-b]thiophene/benzodithiophene (PTB7): PC 70 BM devices ( Figure 6 f). These improvements mostly resulted from increase in short-circuit current density ( J SC ) by enhanced light absorption and forward scattering effi ciency due to the SPR effect of Ag NPs.

Although most reports that use Ag NPs in PSC have used conventional PSCs, a few groups have attempted to develop inverted-type plasmonic PSCs that use Ag NPs. Using Ag NPs between molybdenium trioxide (MoO 3 ) buffer layers as the back-scattering centers, inverted PSCs (iPSCs) have been fabri-cated with the device confi guration ITO/titanium oxide (TiO 2 )/poly(3-hecylthiophene) (P3HT):PC 60 BM/MoO 3 /AgNPs/MoO 3 /Ag, thereby improving PCEs from 2.70% to 3.35%. [ 91 ] Another design exploited the SPR effect by introducing Ag NPs into a TiO 2 buffer layer in iPSCs based on the PTB7:PC 70 BM active layer; optimized concentration of 30% Ag NPs in the TiO 2 layer led to ≈21% increase in PCE (from 6.23% to 7.52%) due to improved exciton generation rate and dissociation probability due to the SPR effect of Ag NPs. [ 92 ]

Recently, a mixture of Ag and Au NPs was introduced into the PEDOT:PSS layer to exploit the dual plasmonic effect of Ag and Au NPs ( Figure 7 a). [ 87 ] These mixed NPs effectively increased light absorption within the whole device architecture compared to those that used only Ag NPs or only Au NPs. The devices with dual NPs exhibited remarkable enhancement in PCEs of PTB7:PC 70 BM devices from 7.25% to 8.67% in comparison to the devices with single Ag NPs (8.01%) and Au NPs (8.16%) ( Figure 7 b).


Table 1. Summary of device characteristics of plasmonic PSCs employing Ag NPs with different nanostructures and locations.

Location Nanostructures Active layer Initial PCE [%]

Final PCE [%]

Refs.

Inside Electrode Ag NPs PTBT:PC 60 BM 3.27 4.31 [6]

PEDOT:PSS Ag NPs PTB7:PC 70 BM 7.90 8.60 [49]

PCDTBT:PC 70 BM 6.40 7.60

Ag NPs PTB7:PC 70 BM 7.25 8.01 [85]

Dual NPs (Ag+Au NPs) PTB7:PC 70 BM 7.25 8.67

TiO 2 Ag NPs (inverted) PTB7:PC 70 BM 6.23 7.52 [81]

Active layer Ag NPs PCDTBT:PC 70 BM 6.30 7.10 [88]

Ag NPs P3HT:PC 60 BM 3.31 3.56 [87]

Ag nanowires P3HT:PC 60 BM 3.31 3.91

Ag NPs PCDTBT:PC 70 BM 5.90 6.40 [88]

Ag nanoplates PCDTBT:PC 70 BM 5.90 6.60

Ag nanoprisms P3HT:PC 60 BM 3.60 4.07 [36]

Dual NPs (Ag NPs+prisms) P3HT:PC 60 BM 3.60 4.30

Ag–Au alloy NPs P3HT:PC 60 BM 3.61 4.73 [57]

Interface ITO/ PEDOT:PSS Ag nanodisks P3HT:PC 60 BM 2.72 3.46 [89]

Ag–SiO 2 core–shell NPs PTB7:PC 70 BM 7.51 8.20 [58]

CD–Ag NPs PTB7:PC 70 BM 7.53 8.31 [90]

PEDOT:PSS/ active layer Ag–SiO 2 core–shell NPs PTB7:PC 70 BM 7.51 8.92 [58]



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J SC

(mA

/cm

2 )

Voltage (V)

-16

-12

-8

-4

0

0.0 0.2 0.4 0.6 0.8

a b c

d e f

Figure 6. Designs of plasmonic PSCs with Ag NPs: a) Schematic illustration of plasmonic PSCs with Ag NPs inside various layers or at the interface between layers. b) J–V curves of PTBT:PC 60 BM devices with NMP:PEDOT:PSS and Ag:NMP:PEDOT:PSS electrodes and c) comparison of enhanced IPCE and absorption by Ag NPs. d) device structure of the devices with size-controlled Ag NPs. [ 6 ] e) simulated ratio of scattering/absorption and forward scattering/total scattering as a function of size of Ag NPs. f) J–V curves of the PCDTBT:PC 70 BM and PTB7:PC 70 BM PSCs with optimized Ag NPs (67 nm in diameter). [ 55 ] Figures reproduced with permission: panel (b) and (c), Copyright 2013, RSC; panel (d–f), Copyright 2013, NPG.

Figure 7. Introduction of Ag NPs into the buffer and active layer: a) Device structure and b) J–V curves of the PTB7:PC 70 BM PSCs with both Ag and Au NPs embedded in PEDOT:PSS layer. c) Device structure and different scattering mechanisms of Ag NPs and Ag nanoplates inside the active layer. d) Comparison of J–V curves in the PCDTBT:PC 70 BM devices with Ag NPs and Ag nanoplates. Reproduced with permission: panel (a) and (b), [ 87 ] Copyright 2013, ACS; panel (c) and (d), [ 93 ] Copyright 2012, RSC.



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Direct contact between Ag NPs and the active layer can induce exciton quenching and recombination loss by non-radiative energy transfer. [ 29,35 ] When applying Ag NPs to an active layer, the surfaces of Ag NPs must be modifi ed, and their size, concentration, and dispersion must be controlled. Ag NPs with different diameters (30, 40, and 60 nm) were synthesized using solution polyol chemistry, then incorporated directly into a bulk heterojunction (BHJ) active layer consisting of PCDTBT and PC 70 BM; [ 88 ] the devices with 40 nm Ag NPs exhibited PCE improvement from 6.3% to 7.1% because the aggregated Ag clusters increased the effi ciency of light trapping within the active layer and improved charge transport from BHJ fi lms to the cathode.

Incorporating Ag NPs into P3HT:PC 60 BM BHJ fi lm improves its structural stability. An Ag NP-free active layer exhibited both bulk reorganization indicated by increase in fi lm thickness, and decreased degradation at the interface between the active layer and the PEDOT:PSS buffer layer under light exposure; in con-trast, the layer with Ag NPs showed only minor degradation at the active/PEDOT:PSS interface. [ 89 ]

Shape-controlled Ag NPs can be applied to the active layer to improve device performance. Embedding 20 wt% of Ag nanow-ires (NWs) in a P3HT:PC 60 BM active layer increased PCEs from 3.31% to 3.91% by improving light absorption and charge-carrier mobilities. [ 90 ] Ag nanoplates blended into BHJ fi lms increase optical path length by effi cient scattering and light trapping in the active layer ( Figure 7 c) [ 93 ] and increased device effi ciencies from 3.2% to 4.4% for P3HT:PC 70 BM, and from 5.9% to 6.6% for PCDTBT:PC 70 BM ( Figure 7 d). When shape-controlled Ag NPs including Ag NPs and Ag nanoprisms were incorporated into the active layer, plasmonic peaks of Ag NPs and nanoprisms appeared at 420 and 600 nm, respectively. [ 39 ] The mixture of two different nanomaterials broadened the absorption spectrum over the whole visible wavelength region (350–750 nm), and optimized P3HT:PC 60 BM devices with Ag NPs and nanoprisms achieved the highest PCEs of 4.30% com-pared to those of the devices only with Ag NPs and Ag nano-prisms (reference: 3.60%, Ag NPs: 3.99%; Ag nanoprisms: 4.07%). Highly monodispersive large Au–Ag alloy NPs (diam-eter of ≈20 nm) with well-controlled composition were applied in a P3HT:PC 60 BM active layer; [ 60 ] optimized Au 11 -Ag 89 alloy NPs doped in the BHJ blend fi lm achieved high PCE of 4.73%, whereas reference devices had PCE of 3.61%. These results were attributed to the alloy NPs acting as scattering centers for increasing light absorption and charge transport and reducing series resistance.

4.2. Silver Nanoparticles at the Interface Between the Layers

The Ag NPs have excellent dispersibility in alcoholic sol-vents, and can therefore be applied to any interfaces within devices such as ITO/PEDOT:PSS and PEDOT:PSS/active layer without removing the underlying layer. To maximize the SPR effect of Ag NPs in PSCs, the distance between Ag NPs and the active layer must be optimized. When this distance is too small, nonradiative decay causes exciton quenching; as the distance increases, the interaction between Ag NPs and active layer decreases exponentially. Therefore, Ag NPs have been

commonly introduced at the interface between ITO and the PEDOT:PSS layer. [ 35,61,88 ] For example, Ag nanodisks were intro-duced into the ITO/PEDOT:PSS interface of P3HT:PC 60 BM PSCs by using electrostatic assembly ( Figure 8 a). [ 94 ] To make a positively charged ITO surface, the ITO substrates were immersed in poly(diallydimethylammonium chloride) solution. This modifi ed ITO surface interacted electrostatically with neg-atively charged Ag nanodisks, leading to increased PCEs from 2.72% to 3.46% ( Figure 8 b).

CD–Ag NPs can be synthesized by irradiating UV light and using carbon dots both as reducing agent and template for Ag NPs (Figure 8 c). [ 86 ] When applied at the interface between PEDOT:PSS and the active layer in PTB7:PC 70 BM PSCs, these CD-Ag NPs resulted in PCE improvement from 7.53% to 8.31%, which was attributed to broad light absorption by ensemble plasmon coupling caused by clustering of Ag NPs on the surface of carbon dots ( Figure 8 d).

Coating of metal NPs with dielectric materials can preserve the SPR effect of metal NPs by avoiding oxidation of metal core under air and by eliminating exciton quenching by direct contact between Ag NPs and active layer. When silica-coated Ag NPs (Ag@SiO 2 ) were introduced at the interfaces both of ITO/PEDOT:PSS (type I) and PEDOT:PSS/active layer (type II) ( Figure 9 a); [ 61 ] the change of Ag@SiO 2 location had strong effect on the dielectric constant of the surrounding matrix, leading to signifi cant differences in electric fi eld distribution and light absorption/scattering effect. PTB7:PC 70 BM devices with type II structure showed higher PCE of 8.92% than the devices without Ag@SiO 2 and with type I (Reference device: 7.51% and type I device: 8.20%) ( Figure 9 b). These results were attributed to strong coupling effect due to close distance between Ag NPs and the active layer, giving rise to additional light absorption and scattering effects over the broad region ranging from 400 to 700 nm ( Figure 9 c). Devices in which bare Ag NPs were used instead of Ag@SiO 2 were fabricated for comparison. The type I devices showed 10% increase in PCE, whereas poor device per-formance was obtained in type II devices. These results imply that silica shells contribute to preventing exciton quenching and that they enable light absorption and scattering that enhance device performance in the type II device structure.

5. Conclusion

The development of new materials and device architectures has contributed to great progress in the effi ciencies of organic opto-electronic devices. Unique properties of Ag NPs enable them to be easily applied inside various layers or at the interface between layers, and their SPR effect effectively increases light trapping or extraction effi ciency and improves energy transfer between donor and acceptor molecules, further enhancing device performances. Although the effi ciencies of plasmonic OLEDs and PSCs are still lower than those of inorganic mate-rial-based devices, Ag NPs may offer the potential to achieve high-effi ciency OLEDs and PSCs by combining active mate-rials and device structures. Basically, attaining extremely high performance of plasmonic OLEDs and PSCs requires develop-ment of novel Ag NPs with optimum size, shape, and compo-sition for broad absorption or extraction and energy transfer




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Figure 8. Introduction of Ag NPs at the interface between ITO and buffer layer: a) Fabrication of electrostatic self-assembled layer of Ag nanodisks and b) J–V curves of P3HT:PC 60 BM PSCs with different size of Ag nanodisks (15 and 30 nm in diameter). c) Real images and schematic illustration for synthesis of CD-Ag NPs and d) J–V curves of PTB7:PC 70 BM PSCs with CD-Ag NPs (inset: device structure with CD-Ag NPs). Reproduced with permis-sion: panel (a) and (b), [ 94 ] Copyright 2011, Elsevier; panel (c) and (d), [ 86 ] Copyright 2013, NPG.

Figure 9. Introduction of Ag NPs at the interface between buffer and active layers: a) Device structures and b) J–V curves of PTB7:PC 70 BM PSCs with type I (ITO/Ag@SiO 2 /PEDOT:PSS) and type II (ITO/PEDOT:PSS/Ag@SiO 2 /PTB7:PC 70 BM) (inset: transmission electron image (TEM) of Ag@SiO 2 ). The diameter of Ag core and thickness of SiO 2 shell are ≈50 and ≈10 nm, respectively. c) Refl ectance spectra of these devices with different locations of Ag@SiO 2 (inset: absorption enhancement by Ag@SiO 2 in the devices with type I and II structure.) Reproduced with permission. [ 61 ] Copyright 2013, ACS.



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enhancement, and effective device architectures for improving device performance.

Acknowledgements S.-H.J. and H.C. contributed equally to this work. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2013R1A2A2A01068753, NRF-2013R1A2A2A01015342). S.-H.J. gratefully acknowledges fi nancial support by Global PH.D Fellowship Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2012H1A2A1002581).

Received: May 28, 2014 Revised: July 20, 2014

Published online:

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